Large-capacity magazine restrictions
As part of expanding the definition of Assault Weapons with the passage of SB-23 in July 1999, prohibitions on magazine capacity sizes were introduced into the Penal Code. SB-23 added a new term, "large-capacity magazine."
Penal Code 12020 (a)(2) states:
12020 (a)(2) Commencing January 1, 2000, manufactures or causes to be manufactured, imports into the state, keeps for sale, or offers or exposes for sale, or who gives, or lends, any large-capacity magazine.
The California Department of Justice Bureau of Firearms FAQ summarizes the restrictions well:
9. If I have a large-capacity magazine, do I need to get rid of it? No. Continued possession of large-capacity magazines (able to accept more than 10 rounds) that you owned in California before January 1, 2000, is not prohibited. However as of January 1, 2000, it is illegal to buy, manufacture, import, keep for sale, expose for sale, give or lend any large-capacity magazine in California except by law enforcement agencies, California peace officers, or licensed dealers. (PC Section 12020 (b)(19-29))
Repairing existing large-capacity magazines
In a letter dated November 10, 2005, Deputy Attorney General Alison Merrilees answered a list of questions about the legality of repairing and replacing parts of legally possessed large-capacity magazines.
The letter clarifies that it is legal to replace parts of a legally possessed large-capacity magazine with parts of any vintage. It also states that the possession, sale, or import of all the parts of an otherwise prohibited large-capacity magazine is not illegal though assembling them into a new large-capacity magazine would be a crime.
The general rules regarding magazine repair appear to be that as long as one ends up with the same number of legally possessed large-capacity magazines, has a plausible path of replacement parts from the original possessed large-capacity magazine to the repaired magazine, and the magazine continues to work in the firearm it was originally designed to operate in, one would not be violating the law.
There are no restrictions on modifying a legally possessed large-capacity magazine to work in another firearm as long as the magazine continues to operate in the firearm it was originally designed for.